Why does inbreeding cause birth defects?


Why does inbreeding cause birth defects?

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When inbreeding there are much higher chances for rare recessive traits to combine in the offspring.

As you might know, if you have one copy you’re typically ok, but if you have two copies then it will express whatever undesirable trait it codes for.


People have two copies of every gene. Some copies are broken, but the second copy makes up for it, so they’re healthy and develop fine. If a developing fetus has two broken copies, sometimes this causes birth defects.

You get one copy of each gene from each of your parents. If your parents are closely related, the chances they both gave you bad copy of the SAME gene is much, much higher than if they’re not related, because they share so many genes to begin with.

Imagine I have a rare defective copy of a gene.

If I have kids with some rando, half of my kids will inherit one defective copy from me – but crucially they’ll inherit a working copy from their mother and will be fine.

But what if I have kids with my cousin, who also has a defective copy?

Now you have a perilous situation: 25% of the kids will inherit *two* defective copies of the gene and *zero* working copies.

Repeated generations of inbreeding make it more likely that these recessive bad copies of genes get carried along – but it only happens if they exist in the first place.

Inbreeding is bad in humans because, in the past, we have had VERY small bottlenecks in our population. I believe that, tens of thousands of years ago, we believe that at one point there were as few as 300 humans on the planet. This means that humans have a very low level of genetic diversity, compared to animals that haven’t experienced a population bottleneck. This lack of genetic diversity makes inbreeding dangerous as malfunctioning, recessive genes can get paired up, result in offspring with diseases.